You can look for Willem Dafoe in Disney’s sci-fi action/adventure film John Carter, but you’ll only hear his distinctive voice coming out of the mouth of Tar Tarkas, the nine-foot-tall, four-armed green Martian who leads his people, the Tharks. He befriends the transplanted Earthling John Carter, played by Taylor Kitsch.

But Dafoe didn’t just record his lines in a booth or shoot his scenes in front of a green screen. He explains how his presence on the set not only helped “human actors” like Kitsch, but also helped his own performance.

Dafoe was attracted to the role principally because of the challenges it offered him as an actor. He explains, “I was excited that John Carter was a big movie and at the prospect of playing a nine-foot-tall Martian warrior. When Andrew [Stanton, director] showed me the designs for the world of the movie, the beauty of it struck me. I liked the cast that was already in place, and I knew the Tars role would be a physically demanding one. I sometimes miss the physical challenges in movie roles but John Carter promised lots of physical performance elements— not only would I perform on three-foot stilts, we had to develop a vocabulary of gesture and the Martian spoken language which is used in part of the film.”

While some directors might have been content to shoot scenes with digital characters like the Tharks separately from the live-action actors, Stanton chose to shoot all the actors together, something which Dafoe praises, saying, “Andrew was very insistent that the Thark character scenes were shot together with the human characters. He felt that with filming, the scenes should be realized with actors performing them, and then they can technically enhance and play with the effects in post. I played my scenes…it was not just fragmented motion capture on green-screen stages. I’m walking around on stilts on location, which gave me the correct Thark height for playing the scenes with Taylor.”

Ultimately Dafoe believes Stanton’s decision made for a better shooting experience, explaining, “It makes a big difference to the human character actors to be reacting to other actors and gives a base and integrity to the reality of the scenes. I’m not sure it has ever been done to this degree before. This film is almost entirely human figures interacting with motion-capture figures.”

While I have to call foul on Dafoe’s last statement there (Andy Serkis would probably want a word with him…), the scenes do play out incredibly well, and Dafoe’s gravelly vocals are sort of what I expect Martians to sound like anyway.